1. Field of the Invention
The invention concerns a thermal turbomachine, in particular an axial flow gas turbine, whose outlet blading is followed by an exhaust casing whose boundary walls essentially comprise a ring-shaped inner part at the hub and a ring-shaped outer part which delimit a diffusor and are connected to each other by a plurality of ribs uniformly distributed over the circumference, the outlet-end bearing arrangement of the turbomachine being arranged in the hollow space within the inner part.
In the case of stationary gas turbine installations, relatively long outlet diffusors are provided, preferably extending in the axial direction, for reasons of fluid mechanics and the better to influence the efficiency. As a result, it is necessary to arrange the exhaust-end turbine runner bearing in the diffusor structure itself. Because of the differing requirements of the diffusor casing, which should be free to expand, and the turbine runners, which cannot be allowed to move about, problems arise which have previously been solved in various ways.
2. Discussion of Background
Thus, for example, in the diffusor of a jet engine gas turbine, which is connected via streamlined ribs to an outer casing, it is known from U.S. Pat. No. 3,261,587 to support the load-bearing ring of the shaft bearing on the outer casing by pins inserted in a thermally mobile fashion in radial planes in the diffusor. Consequently, both the dominant dynamic forces and the expansion forces of the exhaust diffusor are transferred to the outer casing. The outer casing itself is supported on the foundation via claws or suspension supports.
In unsteady operation, as often occurs, for example, in the case of power station gas turbines used for peak-load supply, the casings are subject to thermal stresses and differential expansions which lead to variations in clearance between casing and runner and can also affect the shaft bearing arrangement. Damage to the bearings and stuffing boxes as well as blading damage can then arise. In particular, these disadvantages become effective to a greater extent in today's modern machines with their large dimensions and high temperatures.
An early solution, in which the exhaust diffusor was separated completely from the outer casing of the gas turbine, is known from DE-C-768,036. The outer and inner ring walls of the diffusor are connected together via thermally mobile and expansion compensating elements and the end bearing of the runner is supported in the interior of the inner wall at the hub. Shaft movements can also occur with this type of construction as a result of differential expansions. In particular, large thermal stresses occur at the thermally mobile joints between the exhaust diffusor and the exhaust casing, complicating the sealing problems prevailing there.
In modern machines, the diffusor is no longer itself supported directly on the foundation but is flanged onto the gas turbine casing. The exhaust-end shaft bearing arrangement within the diffusor inner ring is located in an exceptionally hot environment when the machine is operating. Both the positioning of the bearing and the acceptance of the normally vertical load are located in the diffusor structure However, because of the low creep rupture strength of the materials used, the whole load-bearing structure can deform in the course of time.